1. Field of the Invention
The present invention relates to a multicolor printing apparatus in which electrostatic latent images corresponding to respective multiple colors formed on a photosensitive medium by irradiating laser beams are developed and superimposed to form a multicolor image.
2. Description of the Related Art
FIG. 3 illustrates, in block form, a conventional multicolor printing apparatus. In the Figure, reference numeral 51 designates a photosensitive medium, 52 a start hole for recording formed in the photosensitive medium, 53 a recording start sensor comprised of a photo-sensor and operable to detect the start hole 52, 54 a laser diode, 55 a beam drive for driving the laser diode 54, 56 a rotary polygon mirror comprised of six plane mirrors M1 to M6 and a motor 57, 58 a photo-sensor, 59 a scanning start pulse generator operable to generate a scanning start pulse in synchronism with a signal from the photo-sensor 58, 60 an image signal deliverer responsive to an image signal from a host computer, not shown, an the signal from the scanning start pulse generator 59 to deliver an image signal, 61 an F.theta. lens, 62 a timing pulse operable to generate a timing pulse synchronism in with rotation of the motor 57 of the rotary polygon mirror 56, and 63 a rotary polygon mirror drive adapted to control driving of the motor 57 of the rotary polygon mirror 56. The drive 63 includes an oscillator 64 operable to generate oscillating signals at a constant period, a comparator 65 for comparing phases of a signal from the oscillator 64 and a signal from the timing pulse generator 62 and a drive 66 responsive to a signal from the comparator 65 to drive the motor 57 of the rotary polygon mirror 56.
The scanning unit constructed as above operates as will be described below with reference to a timing chart of FIG. 4.
When a start signal (not shown) is first supplied externally to the drive 66, the motor 57 of the rotary polygon mirror 56 starts rotating and a signal in synchronism with the rotation is supplied from the timing pulse generator 62 to the comparator 65 of the rotary polygon mirror drive 63. The comparator 65 then compares phases of the signals from the timing pulse generator 62 and oscillator 64 and sends a difference signal to the drive 66, causing it to drive rotation of the motor 57 of the rotary polygon mirror 56. Thus, the signal from the timing pulse generator 62 is so controlled as to be synchronous with the oscillation period and consequently the rotary polygon mirror can always be rotated at a uniform speed.
Subsequently, the beam drive 55 drives the laser diode 54 and at the same time a laser beam is irradiated on the plane mirror M1 of the rotary polygon mirror 56 and deflected for scanning in synchronism with the rotation of the rotary polygon mirror 56. At the time that a reflected scanning beam passes through the photo-sensor 58 which is so located as to precede the commencement of scanning the photosensitive medium 51, the scanning start pulse generator 59 generates a scanning start pulse as shown at a in FIG. 4.
On the other hand, as the photosensitive medium 51 rotates and the start hole 52 in the photosensitive medium passes by the recording start position sensor 53, a recording start position signal b1 as shown at b in FIG. 4 is generated.
The scanning start pulse a and recording start position signal b are applied to the image signal deliverer 60. After receiving the recording start position signal b1, the image signal deliverer 60 starts delivering an image signal as shown at c in FIG. 4 to the drive 55 in synchronism with the timing for a scanning start signal al reaching t1 time later. Then, the beam drive 55 drives the laser diode 54 to cause it to emit a beam for recording. The beam is scanned by the rotary polygon mirror 56 and passed through the F.theta. lens 61 to form a record of one line on the photosensitive medium 51.
Since the rotary polygon mirror 56 rotates at the uniform speed, the respective succeeding lines are sequentially recorded on the photosensitive medium by means of the plane mirrors M2 to M6 in a similar way, completing a record of one frame.
For multicolor printing, superimposition of data of different colors is needed and therefore, as the photosensitive medium 51 makes a revolution, the passage of the start hole 52 is detected by the recording start position detecting sensor 53 in a similar manner to the previously described image recording process and a recording start position signal b2 is supplied to the image signal generator 60 which in turn starts delivering an image signal as shown at c in FIG. 4 to the beam drive 55 in synchronism with the timing for a scanning start signal a2 reaching t2 time later, thereby completing one frame of printing of the second color in a similar way.
In this manner, superimposition of frames is carried out by the frequency corresponding to the number of recording colors and thereafter a resulting frame is transferred to paper to complete multicolor printing.